International projectionist (Nov-Dec 1933)

10 INTERNATIONAL PROJECTIONIST December 1933 F/LAMENT FILAMENT KH0OSTAT F/LAMENT J1 ' BATTERY-^^ HI6H VOLTAG£ PLATE TRANSFORMER IRON CORE OF TRANSFORMER \MASU — TO 110 VOLT A. c. LINE FIGURE 2 Basic half-wave rectifier circuit current above that value. However, if the filament current is increased, more electrons will be emitted by the filament and a higher plate voltage will be required to reach the saturation current point. An electric current is composed of electrons in motion. The flow of electrons is from the filament to the plate — from a negative to a positive terminal. That is contrary to the popular but incorrect conception that electricity flows from a positive to a negative terminal. That discrepancy is explained by the fact that early experimenters in electricity had no way of determining current flow, thus for convenience they assumed that the current flow was from positive to negative. We now attempt to reconcile those two contradictory ideas by saying that the electron flow is from negative to positive, and the current flow is from positive to negative; but in these articles we will consider that current flows from negative to positive— that is, from filament to plate. Vacuum-Tube Rectifier Current will flow only one way in the plate circuit of a vacuum tube, which is an unidirectional conducting device; so if an alternating current source is connected to the tube in place of the plate battery, current will flow from the filament to the plate only during the time when the plate is at a positive potential with respect to the filament. Incidentally, a 60-cycle alternating current, such as used for house lighting, reverses its polarity sixty times a second. That is, either terminal of the A.C. source is alternately positive and negative sixty times a second, and the two terminals are always of unlike polarity. One complete reversal of polarity represents one cycle, or one wave. Referring to Figure 2, it will be seen that when terminal X of the A.C. line is negative, the pate of the tube is negative, so no current will flow within the tube because the plate, not being positive, will not attract electrons from the filament. But on the half of the cycle when terminal X is positive, the plate of the tube is positive and the tube functions normally. The rectified output is attained from the two leads so marked. By substituting a low-voltage (stepdown), transformer for the filament battery and a highvoltage (step-up), transformer for the plate battery, both operating from the 110-volt A.C. line, the standard half-wave rectifier circuit shown in Figure 3 is obtained. A center tap on the secondary of the filament transformer is used to keep down the amount of hum in the rectified output, which is caused by lighting the filament from A.C. Athough the ends of the secondary winding reverse their poten STEP DQWAt F/LAMENT 'WOW PR/MARV 'WOW PRIMARY TRANSFORMER ' f TO HO VOLT A.C IL.It/S FIGURE 3 Half-wave rectifier circuit tial sixty times a second, the center tap does not vary in potential with respect to the center point of the filament. A seesaw forms a good example of the action that takes place, with the point on which the plank is balanced representing the center tap on the transformer secondary. The secondary voltages of the filament and plate transformers depend on the type of tube employed and the rectified output voltages desired. Full-Wave Rectifier A full-wave rectifier of the type used in sound reproducing systems is illustrated in Figure 4. This circuit receives its name from the fact that it rectifies both halves of the A.C. cycle. For the purpose of explanation, two high-voltage plate transformers are shown, athough actually a single-plate transformer is employed with a tap brought out at the electrical center of the secondary winding. Now, when the terminals of the highvoltage transformers marked X are positive, the plate of tube A is positive and that tube passes current on that half of the A.C. cycle. At the same instant, the plate of tube B is negative, and so that tube does not pass plate current. When the A.C. current reverses polarity on the other half of the cycle, the terminals marked X become negative. Then the plate of tube B becomes positive and that tube passes current ; but the plate of tube A is negative and soit does not pass current. Typical Rectifier Circuit Figure 5 explains diagramatically the functioning of the two rectifier circuits. At A is shown two complete A. C. cycles or waves, (from X to X being one cycle) . It will be seen that the voltage begins at zero, rises to a positive ( + ) valuer falls to zero, rises to a negative ( — ) value, and again falls to zero in one cycle. A half-wave rectifier rectifies only the positive peaks, as shown at B, the negative peaks (represented by the dotted lines), being lost. But a fullwave rectifier rectifies both peaks of the cycle, as illustrated at C. The output voltage of the half-wave rectifier circuit shown in Figure 3 is equal to the secondary voltage of the plate transformer minus the small voltage drop (or loss), through the rectifier tube. The output of the full-wave rectifier of Figure 4 is equal to the secondary voltage of either one of the plate transformers (which must be of exactly the same voltage rating), minus the voltage drop through the corresponding tube. Or if a single, center-tapped plate transformer is employed, the output voltage is equal to one-half the secondary voltage minus the drop through one of the tubes. Filament and plate transformer secondary voltages of 4.5 and 350 are usual with rectifier tubes of medium power, such as the W. E. 205 type. When a vacuum tube is used in this manner, it acts as a "valve" in th& circuit, for it permits current to flow iir TRANSFORMER TRANSFORMERS TO //OVOLT A.C.LINE . FIGURE 4 Full-wave rectifier circuit